What is toxic to humans but useful in absorbing UV radiation in the atmosphere?

Ozone (O3) is indeed toxic to humans at ground level, where it can cause respiratory issues and other health problems due to its reactive nature. However, in the stratosphere, it plays a crucial role in absorbing harmful ultraviolet (UV) radiation from the sun. This absorption is vital for life on Earth, as it protects living organisms from the damaging effects of UV rays, which can cause skin cancer and harm ecosystems.

The formation of ozone occurs naturally through the reaction of ultraviolet light with molecular oxygen (O2), leading to a beneficial ozone layer that screens out the majority of the sun's harmful UV radiation. This distinction between its effects in the atmosphere—where it serves a protective function—and at ground level, where it can be harmful, underscores the importance of ozone in the environmental context.

In contrast, other options do not serve as effective UV absorbers in the atmosphere. Carbon monoxide is a product of incomplete combustion and contributes to air pollution, while nitrogen dioxide is primarily known for its role in smog formation. Chlorine gas is indeed toxic and used industrially, but it does not have a role in UV absorption like ozone does. Thus, ozone is the unique compound among the choices provided that fulfills the protective function while also